I encountered this problem as well, including the ability to hack around it by changing permissions of /usr/libexec/dbus-daemon-launch-helper.

It took a while, but I eventually tracked down my actual problem. Somehow the main GID number for the messagebus user in /etc/passwd was not consistent with the messagebus GID number in /etc/group. Most likely it was messed up because I had copied parts of both files to maintain real user's IDs with other machines, which may have interacted poorly with scripts to add the system IDs. But oddly, I did that copying more than a year before this problem manifested itself after an update.

Fixing the GID in /etc/passwd fixed the problem without requiring hacked up permissions on /usr/libexec/dbus-daemon-launch-helper.

Fixing the GID in /etc/passwd fixed the problem without requiring hacked up permissions on /usr/libexec/dbus-daemon-launch-helper.

re-emerge dbus and reboot fixes the GID. I encountered this years ago, and now I see it is still not fixed. When building a new stable branch system it gets unknown GID 99 while messagebus GID in my case was 105._________________best regards/ pozdrawiam
PiotrAF